Process for machining a spur gear by means of a rotating gear-like tool

ABSTRACT

In a process for machining a spur gear by using a grinding worm having a curved tooth thickness smaller than the final measurement of the curved tooth gap of the spur gear to be produced, the grinding worm is first directed radially towards the spur gear until the desired distance between the axes is reached. Subsequently, a relative circular feed movement is carried out consisting of a positive or negative additional rotating movement of the gear or the worm. This additional rotating movement is superimposed on the corresponding basic revolution of the gear or the worm. In this way, first one flank and subsequently the other flank of each gear tooth is machined to its final dimension. With this process, a full linear contact exists between the grinding worm and the tooth flank of the gear during roughing independent of the amount of material to be machined. Due to the existing play, an effective cooling is possible so that high grinding outputs can be achieved.

This is a continuation of application Ser. No. 342,960, filed Jan. 26,1982, now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to a gear machining process wherein arotating gear-like or worm-shaped tool is radially moved into engagementwith a gear-type workpiece with the shape of the teeth of the tool andworkpiece being such as to prevent engagement of said teeth until asubsequent relative circular feed movement is performed in oppositedirections to machine first one flank of the workpiece teeth and thenthe other flank thereof.

In known processes of continuous gear machining by means of immersionscraping, immersion grinding and immersion honing of cylindrical spurgears, the workpiece is engaged with a gear-like, rotatably drivenscraping or honing tool having a globoidal or hyperboloidlike shape insuch a way that the desired gear geometry is obtained on the workpiecewith a radial advance movement between the tool and the workpiece whenthe distance of the axes of the tool and the workpiece, which arecrossed towards each other, has reached the final value or the desireddistance between axes which corresponds to the profiles of the tool andthose of the finished machined workpiece.

An advantage of this process, for example, in comparison with the wellknown continuous gear rolling grinding with a cylindrical grinding worm,is seen in the fact that the desired workpiece geometry can be producedby a single movement between tool and workpiece, i.e. the immersionfeed, since the line of engagement between the tool and the workpieceextends across the entire width of the workpiece when the rated axisdistance (desired distance between axes) is reached and that a highmaterial removing capacity can be reached due to the great length of theengagement with the workpiece. Since an additional axial machining feedis thereby eliminated, the process is very economical. Such a knownprocess, as described above is covered by the German DisclosurePublication No. 25 16 059 which discloses the use of a rotating,gear-like tool for the production or machining, particularly for thegrinding, of the teeth of straight or oblique spur gears. The toolconsists of a grinding worm in the described embodiment.

As is generally known, no definition-bound difference is made between agear and worm in gear technology. The latter represents, in thetechnical sense, an oblique gear which may have a small number of teeth,even one. For this reason, the term "gear-like tool" comprises worms aswell as other gears which may have teeth inside and outside. Themachining can be effected through chipping, removing and also reshaping.

The prerequisite for the grinding with only one radial advance movementis the covering of the teeth of the tool with those of the workpiece,i.e. the joint tooth engagement must extend from the one face of theworkpiece to the other whereby the tool has a hyperboloid ofgloboidal-like shape. The tool, in this instance the grinding worm, mustbe circumferentially shaped in such a manner that it forms a correcttooth engagement with the teeth of the workpiece when the ratedproduction axis distance is reached. In case of tooth corrections, thetool must be provided with these corrections, for example, correctionsfor tip relief profile ease-off and base relief profile ease-off.

The machining of the workpiece according to the known process of purecutting-in has a number of disadvantages. At the beginning of themachining, the workpiece has a more or less large dimension which is tobe machined, i.e. there is a deviation between the real shape of theworkpiece and the shape which the workpiece will have at the end of theimmersion machining and to which the tool shape is coordinated.

This discrepancy between tool and workpiece shape, which will onlydisappear with the removal of the undesired portions of the workpiece,i.e. when the machining process is terminated, has the effect that, atthe beginning of the machining, very unfavorable engagement conditionswith unsymmetrical, point-shaped engagements exist depending on theheight of the workpiece to be machined which run counter to a highmaterial removing capacity. This means that this process is onlysuperior to other processes with workpieces having a relatively smallamount to be machined which requires a correspondingly high precision ofthe preliminary machining.

Furthermore, the non-uniform material removal leads to undesirableburned spots with a fast feed. This is particularly true because thecooling is insufficient between the tool and the workpiece uponimmersion since the teeth of the tool fully fill the tooth gaps of theworkpiece and there is no space for the cooling agent.

SUMMARY OF THE INVENTION

The present invention provides a new and improved process by which theabove-mentioned disadvantages are avoided. With the present process isis possible to effect the machining of the workpiece in such a fashionthat, even with a relatively large dimension to be machined, a linearengagement is possible between the tool and the workpiece therebyachieving optimum chip removal during the full period of the machiningoperation. Furthermore, the cooling possibilities with the presentprocess are improved between the tool and the workpiece.

The present invention provides a new and improved process for machiningthe teeth of a rotating gear workpiece by means of at least one rotatingworm-shaped tool which can be moved toward engagement with the workpieceand upon reaching the desired distance between the axes of the tool andworkpiece will rest against the workpiece with the ratio of therevolutions corresponding to a basic revolution ratio resulting from theratio of the number of teeth of the workpiece and tool, comprisingradially feeding said tool having a curved tooth thickness smaller thanthe final measurement of the width of the curved tooth gap of theworkpiece to be produced, relative to the said workpiece until thedesired distance between said axes is reached and subsequentlyperforming a relative circular feed movement consisting of a positiveand negative additional rotating movement of one of said workpiece ortool which is superimposed on the corresponding basic revolution so thatfirst one flank of the teeth of said workpiece and, subsequently, theother flank of said teeth are machined.

The present invention is also directed to a new and improved apparatusfor carrying out the foregoing process wherein drive means are providedfor rotating the tool or workpiece, feed drive means are provided forrelative feeding and return movements between the workpiece and the tooland additional drive means are provided to produce an additionalrotating movement of the workpiece or the tool.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the engagement conditions between aspur gear and a grinding worm according to a prior art immersion processdescribed herein.

FIG. 2 is a schematic view showing the engagement conditions with theprocess according to the present invention.

FIG. 3 is an enlarged detailed view of a portion of FIG. 2.

FIG. 4 is a schematic diagram showing the individual steps of theoperation for machining a spur gear.

FIG. 5 is a schematic view of the apparatus for the implementation ofthe process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The unfavorable engagement characteristics as caused by the pureimmersion grinding of a cylindrical spur gear 1 by means of agloboidal-shaped grinding worm 2 are shown in FIG. 1. The finishedprofile of the gear to be produced is indicated at 3 and the crude toothflanks of the workpiece are indicated at 4. The profile shape 5 of theknown worm 2 is designed in such a fashion that the exact finishedprofile 3 of the gear 1 develops when the rated production axis distanceis reached between the workpiece and the tool whereby possible toothcorrections, such as tip relief profile ease-off and base relief profileease-off as well as longitudinal crowning, are taken into consideration.

At the beginning of the immersion process in FIG. 1, no linear contactis initially developed but only a one or two-point contact as indicatedat the circled points 6. The desired favorable conditions of the profilerolling grinding, i.e. the linear contact, only occur when the finisheddimension of the gear is achieved, i.e. when the grinding worm 2 hasbeen radially introduced by the entire amount x. The prior art processhas this characteristic because the engagement line is not a straightone but a line or area curved in space. (It should be pointed out thatonly the involute tooth system has the feature of correct engagementwith a straight line as the engagement line, even with a changingdistance between the axes.) Therefore, there is only a point contact inall phases of preliminary grinding. Thus, considerable heating isproduced even with a reduced quantity of metal being removed and, as aresult, burned spots occur on the finished ground gear.

If, in accordance with the present invention as shown in FIGS. 2 and 3,the profile 7 of the grinding worm 2 is selected in such a manner thatits curved tooth gap L_(s) is larger or at least equally as large as thecurved tooth thickness D_(z) of the gears, i.e. L_(s) ≧D_(z) ; or,expressed in an analogous manner as a function of the curved tooththickness D_(s) of the tool and of the curved tooth gap L_(z) of thegear: D_(s) ≧L_(z), then the grinding worm 2 can be immediately moved tothe full depth until the rated production axis distance "a" is reachedbetween the grinding worm axis 8 and the spur gear axis 9 withoutincurring contact with the tooth flanks 4 of the workpiece which arestill raw. At this point, the rotating tool 2 and the rotating workpiece1 are in the correct position for tooth engagement whereby the ratio ofthe revolutions corresponds to a basic revolution ratio resulting fromthe ratio of the two numbers of teeth of the workpiece and tool.

Now, a relative circular advance motion is carried out between theworkpiece and the tool which consists of a positive and subseqently anegative additional rotating movement of the workpiece or of the tool,which additional rotating movement is superimposed on the correspondingbasic rotation of the workpiece or the tool. In this way, first the oneand then the other tooth flanks of the workpiece are successivelyground.

The transfer motion program for the machining is shown in FIG. 4 and isbroken down into the following steps wherein the first step is indicatedby the transfer step symbol 10:

Transfer Step 10--The grinding worm of tool meshes with the workpiece tothe full depth, i.e. a feed movement is effected from the center of thepre-machined tooth gap of the workpiece, until the rated production axisdistance "a" is reached. No machining takes place during step 10.

Transfer Step 11--The workpiece is now moved through an angle ofrotation ω_(0R) at a rapid traverse rate until the tooth flank almosttouches the grinding worm flank. This additional rotating movementthrough the angle ω_(0R) is superimposed on the basic rotation of theworkpiece, i.e. its rotation is somewhat accelerated. The amount of thedead angle of movement ω_(0R) could be measured, for example, by acontact sensor.

Transfer Step 12--Now the actual machining of the right tooth flanktakes place beginning with the rough finishing which corresponds to arelative rotation ω_(1R).

Transfer Step 13--The right flanks are subjected to final machining orsmooth finishing through the range of the angle of rotation ω_(2R).

Transfer Step 14--In the righthand final position which corresponds toan angle of rotation ω_(R), the rotating advance ceases for the durationt_(R) during one of several workpiece rotations for the prupose ofgrinding-out and sparking-out.

Transfer Step 15--This step comrpises a reverse rotation through thecenter position and until contact is made of the lefthand tooth flank ofthe workpiece with the corresponding grinding worm flank. Overall, thereverse rotation through the dead angle of movement ω_(0L) takes placeat a rapid traverse rate. The rotation of the workpiece is, accordingly,somewhat delayed.

Transfer Steps 16 to 18--In a manner analogous to the righthand flanks,the lefthand tooth flanks are now rough ground and smooth ground(rotating angles ω1L and ω_(2L)) whereupon the rotational advance ceasesat the lefthand final position ω_(L) whole workpiece rotation takesplace during the period t_(L).

Transfer Step 19--The workpiece is now accelerated through the angle ofrotation W_(M) until the central position is reached for the removal ofthe tool.

Transfer Step 20--The last operational step comprises the radialseparation of the tool and workpiece.

In a specific machining example, the grinding worm was given such aprofile that the raw workpiece fits into the grinding worm profile witha total rotational play of 0.2 mm, i.e. 0.1 mm play "s" per side. Theraw gear has a total machining dimension of 2b=0.4 mm in the spur cut;this means that a machining dimension "b" of 0.2 mm is to be ground offon each flank. As a result, with the grinding worm at full depth, theworkpiece, measured at the circumference, must be rotated in eachdirection by 0.3 mm relative to its central position so that the desiredcurved tooth finished dimension is reached.

This corresponds, when expressed as a function of the pitch radius r_(z)of the workpiece, to a rotation of the latter by

    Δω=(s+b)/r.sub.z

The play "s" and the machining dimension "b" per tooth flank is measuredin the spur cut and in the circumferential direction of the gear.Instead of the rotation of the gear (workpiece), an equivalent rotationof the grinding worm can also be provided.

For the trimming of the grinding worm, a diamond gear is used whosecurved tooth thickness, measured on the pitch circle, is smaller than orequal to the desired width of the curved tooth gap of the grinding worm.Also the diamond gear is initially introduced to the full depth andthen, proceeding from the central position, is rotated once in thepositive and once in the negative sense of rotation by an angle of Δλ.This additional rotating movement is again superimposed on the basicrotating motion of the diamond gear, i.e. it is mutually somewhataccelerated and subsequently somewhat delayed.

In this way, a diamond gear is simulated which has a curved tooththickness in the spur cut larger by 2·Δλ·r_(d) than actually exists.(r_(d) indicates the pitch radius of the diamond gear.) Δλ isexpediently selected in such a way that the desired curved tooth widthdevelops on the grinding worm. In general, the two additional rotatingangles Δω (for the gear machining) and Δλ (for the trimming with thediamond gear) are not of the same size.

The new process according to the present invention has the followingadvantages in comprison with the processes used previously:

(1) There is a full linear contact during roughing between the grindingworm and the tooth flank of the gear independent of the materialsurplus.

(2) There is no dependence anymore between the curved tooth thickness ofthe ground gear and the curved tooth thickness of the trimming gear. Thedesired curved tooth thickness of the workpiece is simply set on thecontrol of the machine-tool which will be described below. Due to thepossibility of the re-lapping of the diamond gears without influencingthe flank diameter of the gears to be ground, the diamond gears have alonger service life.

(3) The grinding worms can be used for longer periods since the toothflanks can be trimmed several times before a new depth must be set.

(4) Finally, there are more effective cooling possibilities due to theexisting play so that very high grinding output can be achieved.

A system for the performance of the process according to the inventionis shown in FIG. 5. This system has an input unit 21 for the tool andworkpiece parameters as well as for the machining data. A feed generator2 supplied by the input unit 21 acts on a bearing circuit 23 to producethe forward and return motion of the tool carriage 24 on which thegrinding tool 2 rests. The bearing circuit 23 is comprised of thecontroller 25, the servo-amplifier 26, the carriage advance motor 27 andthe linear measuring system 28.

The feed generator 22 is also connected with a circuit 29 to produce therotating motion of the workpiece 1. This circuit 29 is comprised of thecontroller 30, a servo-amplifier amplifier 31, the workpiece motor 32and a rotating angle transmitter 33. The signals of the feed generator22 and of the rotating angle transmitter 35 which is connected with thegrinding motor 34 form together the rated value of the workpiecerotation.

When the rated production axis distance "a" is reached, the globoidal orhyperboloid-shaped grinding worm 2 comes to rest closely against thegear 1. Depending on the angle of inclination and direction of tool andworkpiece, the axis crossing angle can amount from 0° to 90°.

The following data are entered into the input unit 21: the thread ortooth numbers of the tool and of the workpiece, the curved tooththickness of the profiling tool, the rated curved tooth thickness of theworkpiece, the rated axis distance between workpiece and tool, the feed,roughing and smoothing paths in the feed direction of the tool carriagefor the preliminary profiling of the tool or the preliminary machiningof the pre-toothed workpiece. Furthermore, there are entered into theinput unit: the speeds in the individual feed or cross-feed phases, thefeed, roughing the smoothing angles and feed, roughing and smoothingangle speeds for the additional rotating movement of the tool or of theworkpiece as well as dwell times after the termination of the machiningfeed movements.

Instead of driving the tool and the workpiece with separate motors,there could also be a joint drive. In this case, a differential gearwould have to be used in order to divert the workpiece rotating movementfrom the tool rotating movement and rotating movement of the feed motor.

Another possibility consists of driving only the tool which, in itsturn, drives the workpiece through the tooth engagement whereby theadditional rotating movement is then produced by means of a brakingtorque on the workpiece. Instead of driving the tool, only the workpiececould be driven in an analogous manner.

In a particular advantageous design, both tooth flanks of the workpiececan be simultaneously machined with two tools which, for example, arediametrically opposite each other, whereby the circular feed movement iseffected by means of additional rotating movements of the tools. Whenthe workpiece is subject to lower requirements, its drive an beeliminated whereby the rotation of the workpiece is effected by thetools through the tooth engagement. The workpiece is, in this case,"clamped" in a certain way between the tools.

The machining process according to the invention is not restricted tothe grinding of spur gear with a grinding worm. It covers quitegenerally the machining of workpiece by means of a cutting, removing orre-shaping profiling or trimming tool. If the workpiece is not to bemachined in the bottom of the gap, the base circle diameter of theprofiling tool must be larger than that of the workpiece.

The following possibilities are, for example, under consideration:

(1) The tool has a globoidal or hyperboloid-like shape dependent on theexternally toothed workpiece to be machined and is a threaded orworm-shaped grinding or honing wheel provided with external toothing.

(2) The tool has a barrel shape dependent on the internally toothedworipiece to be machined and is a threaded or wormshaped grinding orhoning wheel provided with external toothing.

(3) The tool has a globoidal or hyperboloid-like shape dependent on theexternall toothed workpiece to be machined and is an externally toothedscraping or honing gear wheel.

(4) The tool has a globoidal or hyperboloid-like shape dependent on theexternally toothed workpiece to be machined and is an internally toothedscraping or honing gear wheel or an internally toothed grinding worm.

(5) The tool has a barrel shape dependent on the internally toothedworkpiece to be machined and is an externally toothed scraping or honinggear wheel.

(6) A cylindrical or crowned dressing gear wheel is used for thedressing of the tool with a cutting, removing or reshaping surfacewhereby the shape of this dressing gear wheel corresponds to the desiredgeometry of the tool to be produced with the exception of the widthwhich is either somewhat larger or somewhat smaller in the case of theprofiling tool and is simulated as a larger width by an axial shiftingof the profiling tool during profiling with a simultaneous rotationconsidering the angle of the tooth.

The process according to the invention is also suitable for theproduction of a workpiece which has not been preliminary toothed. Forthis purpose, the tool and the workpiece are moved in radial directiontowards each other while removing or dislodging material until the ratedaxis distance is reached whereupon the continued machining or finalmachining is effected by means of the above described relative circularfeed movements.

While the invention has been particularly shown and described withreference to preferred embodiments thereof it will be understood bythose in the art that the foregoing and other changes in form anddetails may be made therein without deparing from the spirit and scopeof the invention.

What is claimed is:
 1. A process for machining the teeth of a rotatinggear workpiece by means of at least one rotating worm-shaped tool whichcan be moved toward engagement with the workpiece and upon reaching thedesired distance between the axes of the tool and workpiece will restagainst the workpiece with the ratio of the revolutions corresponding toa basic revolution ratio resulting from the ratio of the number of teethof the workpiece and tool, comprising radially feeding said tool havinga curved tooth thickness smaller than the final measurement of the widthof the curved tooth gap of the workpiece to be produced relative to saidworkpiece until the desired distance between said axes is reachedwithout contact and subsequently performing a controlled relativecircular feed movement comprised of an additional rotating movement ofone of said workpiece and tool which is superimposed on thecorresponding basic revolution so that at least one flank of each toothof said workpiece is machined, said circular feed movement beingcontinously measured during the machining operation and stopped uponreaching a predetermined feed angle.
 2. A process according to claim 1further comprising machining said one workpiece flank during a roughingfeed increment, a smoothing feed increment and a feed dwell at the finalposition during at least one workpiece rotation and subsequentlyreversing and analogous machining the other workpiece flank.
 3. Aprocess according to claim 2 wherein the roughing and smoothing feedincrements are effected continuously.
 4. A process according to claim 1wherein said additional rotating movement of the tool and of theworkpiece is implemented by means of an electronic control andregulating system whereby the tool and the workpiece are each driven bya separate motor.
 5. A process according to claim 1 wherein both flanksof the workpiece teeth are machined simultaneously by using two toolswhereby circular feed movement is effected by means of additionalrotating movements of said tools.